A program for the development of new agents to treat T. Cruzi, the parasite responsible for Chagas' Disease, is presented. Cruzain, a cysteine protease is identified as a target for a structure based drug discovery program. Models of the three-dimensional structure of cruzain built by homology to other cysteine proteases of known structure and the recently determined x-ray structure of cruzain will be used as a template for the in compuo screening of the fine chemicals directory, a data base of about 70,000 purchaseable small molecules. Compounds selected in this computer based screen will be characterized experimentally. We expect that 2-10% of the computer selected inhibitors will be active experimentally against the enzyme at concentrations less than 100 microM. Already, we have identified cruzain inhibitors active at 2-4 microM concentrations. Free energy perturbation calculations will be used to help the synthetic chemists develop more active compounds. Some will be based on the non-peptidic leads derived from the computer data base searches, while others will be analogs of the nanomolar mechanistic inhibitor Z-Phe-Arg-FMK (Z= carbobenzoxy, FMK = fluoromethyl ketone) that have been altered to eliminate peptide character. A close interaction between molecular biology, structural biology, synthetic chemistry and computational chemistry will be used to exploit a structure based drug design cycle.